1. Field of the Invention
The present invention generally relates to a time division multiplex transmitting/receiving system. More specifically, the present invention is directed to a time division multiplex transmitting/receiving system in which the transmitting frequency thereof is different from the receiving frequency thereof, and the transmission and the signal reception are switched for every predetermined time slot.
2. Description of Prior Art
In the GSM (Group Special Mobile) corresponding to the digital cellular system in Europe, as shown in FIG. 1, a service area is subdivided into a plurality of regions "Ra" to "Re". To the respective subdivided service regions, exclusive-use frequencies "fa" to "fe" are allocated as frequencies of transmission/reception signals. These exclusive-use frequencies are: determined in such a manner that the frequencies of the adjoining regions must be separated from each other as much as possible.
As shown in FIG. 2(a), the frequencies "fa" to "fe" are constituted by the transmission frequencies and the reception frequencies, and as the transmission (TX) frequencies, the frequency band of a range from 890 MHz to 915 MHz is employed. This frequency band is subdivided into 125 channels (CH) with a bandwidth of 200 KHz respectively. As shown in FIG. 2(b), the reception (RX) frequencies are set to a frequency range from 935 MHz to 960 MHz. This frequency band is also subdivided into 125 channels with a bandwidth of 200 KHz, respectively.
The GSM utilizes a so-called "frequency hopping" to establish a privacy function. That is, the respective channels are segmented into frames for every predetermined time along the time axial direction, 4.616 ms in the GSM. Considering a single mobile station, the channel (frequencies) used by this mobile station is changed for every frame. It should be noted that the transmission/reception channels (frequencies) are selected in such a manner that a difference between the transmission channel frequency and the reception channel frequency is continuously 45 MHz.
Each of the frames is subdivided into 8 time slots (namely, a single time slot being 0.577 ms), as illustrated in FIG. 3. Among these 8 time slots, a preselected one time slot (namely, the head slot in the example of FIG. 3) is determined as a reception (RX) time slot. Only a time slot subsequent to this reception (RX) time slot by 3 time slots is used as a transmission (TX) time slot. A time slot succeeding to this transmission-(TX) time slot by either two, or three time slots is used as a monitoring (MON) time slot. In this monitor (MON) time slot, a strength of an electric field of a signal transmitted from the adjoining base station is monitored. When the field strength of the reception signal from the adjoining base station becomes higher than a predetermined field strength, the reception channel is switched to this adjoining base station. It should be noted that although the transmission (TX) time slot is synchronized with the reception (RX) time slot, the monitor (MON) time slot is not always synchronized with these transmission (TX) and reception (RX) time slots. The channels to be monitored are changed for each frame. As a result of channel monitoring, empty channels with better reception conditions are properly selected, and then utilized as the transmission and reception time slots time sequentially succeeded to the empty channels.
FIG. 4 represents a terminal unit employed in such a GSM, namely one example of an arrangement of a transmitting/receiving apparatus mounted on an automobile, or portable by an operator. As shown in FIG. 4, this apparatus includes an RF (radio frequency) unit 1 connected to an antenna, and a baseband process unit 2. The RF unit 1 is coupled with the baseband process unit by way of A/D converters 3a, 3b and D/A converters 4a, 4b.
A signal received by the antenna 11 is separated from a transmission system (i.e., system constructed of blocks having reference numerals of a series of 30 shown in FIG. 4) by a duplexer 12 and outputted to a reception system (i.e., system constructed of blocks having reference numerals of a series of 10 and 20). An RF (radio frequency) low-noise amplifier 13 is positioned at a front end of the reception system to amplify this reception signal. The output of this RF low-noise amplifier 13 is inputted into a reception channel band-pass filter 14. As previously explained, the frequency bandwidth of the reception signal are 936 MHz to 960 MHz. The reception channel band-pass filter 14 separates the frequency component of the presently selected predetermined reception channel, and then outputs the separated frequency component to a first-stage reception mixer 15.
An oscillator 41 outputs a signal (carrier) having a frequency corresponding to either the reception channel, or the transmission channel in the range from 1006 MHz to 1031 MHz. The frequency of the signal outputted from this oscillator 41 is controlled in such a manner that this output frequency becomes higher than the frequency of the reception channel signal by 71 MHz. Since the first-stage reception mixer 15 mixes (multiplies) the reception signal supplied from the reception channel band-pass filter 14 with the signal outputted from the oscillator 41, an output from this mixer 15 contains a frequency component of 71 MHz corresponding to a difference between the frequencies of both these input signals. In other words, the reception signal is converted into a first intermediate frequency signal having a frequency of 71 MHz.
A first-stage intermediate frequency filter 16 separates this first intermediate frequency signal of 71 MHz from the output signal of the first-stage reception mixer 15. A first-stage intermediate frequency amplifier 17 amplifies this separated signal and supplies the amplified signal to a second-stage reception mixer 18. An oscillator 42 supplies a signal having a frequency of 65 MHz to the second-stage reception mixer 18. Since the second-stage reception mixer 18 mixes (multiplies) the first intermediate frequency signal of 71 MHz supplied from the first-stage intermediate frequency amplifier 17 with the signal having the frequency of 65 MHz outputted by the oscillator 42, the first intermediate frequency signal having the frequency of 71 MHz is converted into the second intermediate frequency signal having the frequency of 6 MHz.
The second intermediate frequency filter 19 separates this second intermediate frequency signal having the frequency of 6 MHz and supplies the separated signal to a second-stage intermediate frequency AGC amplifier 20. The second-stage intermediate frequency AGC amplifier 20 controls the gain of this second intermediate frequency signal in response to a control signal supplied from the baseband process unit 2 and supplies the gain-controlled second intermediate frequency signal to a quadrature demodulator 21. The quadrature demodulator 21 quadrature-demodulates the second intermediate frequency signal supplied from the second-stage intermediate frequency AGC amplifier 20 with employment of a reference signal having a frequency of 6 MHz outputted-by an oscillator 44, thereby outputting an I-component signal and a Q-component signal. The I-component signal and the Q-component signal are A/D-converted by the A/D converters 3a and 3b, respectively, and the A/D-converted signals are supplied to the baseband process unit 2. The baseband process unit 2 processes the I-component signal and the Q-component signal to reproduce the original signal, i.e., the voice (audio) signal which will then be supplied to a speaker or the like (not shown).
On the other hand, when a signal is to be transmitted is, a voice (audio) signal inputted from a microphone (not shown) is processed by the baseband process unit 2 to produce the processed signal to be transmitted. An I-component signal and a Q-component signal in this processed signal are D/A-converted by the D/A converters 4a and 4b and the D/A-converted signals are inputted into a quadrature modulator 31. The quadrature modulator 31 quadrature-modulates these I-component signal and Q-component signal with using a signal having a frequency of 116 MHz outputted by an oscillator 43. Signal components of the signal derived from the quadrature modulator 31 other than the signal component having the frequency of 116 MHz are removed by a transmission intermediate frequency filter 32, and then are amplified by a transmission intermediate frequency amplifier 33. Thereafter, the amplified signal is inputted into a transmission mixer 34.
To this transmission mixer 34, the same signal as that supplied to the first-stage reception mixer 15 in the signal reception system is supplied from the oscillator 41. As previously described, the frequency of this signal is selected to be higher than the frequency of the reception signal by 71 MHz. The frequency of 116 MHz for the signal inputted from the quadrature modulator 31 into the transmission mixer 34 is selected to be higher than the frequency of 71 MHz for the first intermediate frequency signal outputted from the first-stage reception mixer 15 by 45 MHz. As a consequence, the frequency of the transmission signal derived from the transmission mixer 34 which mixes the signal having the frequency of 116 MHz supplied from the transmission intermediate frequency amplifier 33 with the signal supplied from the oscillator 41, contains a lower frequency component than the frequency component of the reception signal inputted into the first-stage reception mixer 15 by 45 MHz.
A transmission channel band-pass filter 35 separates from the signal outputted from the transmission mixer 34, only a component corresponding to the transmission channel frequency band, i.e., the frequency component lower than that of the reception channel by 45 MHz, thereby outputting the separated signal component to a power amplifier 36. The power amplifier 36 power-amplifies the inputted signal and then supplies the power-amplified signal to the duplexer 12 via an isolator 37. The duplexer 12 transmits the inputted signal through the antenna as the electromagnetic wave.
FIG. 5 represents a frequency relationship of the signals inputted/outputted into/from the respective circuit blocks within the apparatus shown in FIG. 4. As shown in FIG. 5, the oscillator 41 is a variable frequency oscillator for producing a signal having a predetermined frequency which is varied from 1,006 MHz to 1,031 MHz at a step of 200 KHz. The oscillators 42 to 44 correspond to fixed frequency oscillators for producing a signal having a constant frequency of 65 MHz, 116 MHz, or 6 MHz.
These oscillators 41 to 44 employed in the RF unit 1 are arranged by a so-called "PLL synthesizer", namely by a voltage controlled oscillator (VCO), a low-pass filter, a phase comparator, and a frequency divider.
FIGS. 6(a) and 6(b) indicate variations in the frequencies of the signals outputted from the oscillator 41. As represented in FIGS. 6(a) and 6(b), the oscillating frequency of the oscillator 41 is set to a frequency "f.sub.1 " corresponding to the reception channel (transmission channel) in the reception (RX) slot within the frame. Since the oscillating frequency is also used in the transmission (TX) time slot succeeding to the reception (RX) time slot by 3 time slots, the frequency of the oscillator 41 is fixed until this time. Then, after the transmission (TX) time slot is ended, this frequency of the oscillator 41 is set to a frequency "f.sub.2 " corresponding to the reception channel to be monitored in another time slot to be monitored.
That is to say, after the transmission time slot is completed, the oscillator 41 starts its scanning operation to thereby change the oscillating frequency from "f.sub.1 " into "f.sub.2 ". Then, after the monitoring operation is ended, the oscillator 41 restarts its scanning operation, so that the oscillating frequency of "f.sub.2 " is changed into another frequency "f.sub.3 " corresponding to the reception channel in the reception time slot within the subsequent frame. As previously explained, the oscillator 41 scans its oscillating frequency at a high speed during a time period from approximately 0.5 ms to 1 ms.
According to the GSM recommendation, the interference characteristic of adjoining channels (C/I ratio) in the signal reception system is defined as represented in FIG. 7. In other words, the C/I ratio of -9 dB is required for an adjoining channel separated from the desirable channel of 200 KHz (i.e., 1 channel); the C/I ratio of -41 dB is required for another adjoining channel separated from the desirable channel by 400 KHz (=2 channels); and furthermore the C/I ratio of -49 dB is required for another adjoining channel separated from the desirable channel by 600 KHz (=3 channels). Even when the levels of the interference waves are higher than the levels of the desirable waves, the desirable waves must be received.
As a consequence, when all of the RI unit 1 would be arranged by analog signal systems, even if both the desirable waves and the interference waves could be received at the same levels, the band limit of -9 dB is required for the frequency separated from the desirable wave by 200 KHz, the band limit of -41 dB is required for the frequency separated from the desirable wave by 400 KHz, and the band limit of -49 dB is required for the frequency separated from the desirable wave by 600 KHz at the stages of the intermediate frequencies. As a result, in the apparatus shown in FIG. 4, an SAW filter is employed as the first-stage intermediate frequency filter 16 for separating the first intermediate frequency signal having 7 MHz. A ceramic filter is employed as the second-stage intermediate filter 19 for separating the frequency of 6 MHz.
To improve portability of the GSM terminal unit, this terminal unit should be made compact and light weight as permitted as possible, and also low power consumption should be achieved. As a result of great improvements in the recent digital signal processing techniques and the recent very large scaled digital IC technology, the baseband process unit 2 could be made compact and powered under low consumption. Although the RF unit 1 would be manufactured by IC and made compact under low power consumption, since this RF unit 1 employs the oscillators and the filters, these circuit components are relatively difficult to be made by IC. Also it is a practical reason that as these filters and oscillators employ such components for requiring adjustments, they are not easily made compact and operated under low power consumption, As a consequence, it is desirable for the RI unit 1 that, for instance, the total number of the stages for the intermediate frequency signal processing, and also the quantities of local oscillators and filters would be lowered in order to improve compactness and lower power consumption thereof.
Then, it would be conceivable that for instance, the apparatuses shown in FIG. 4 and FIG. 5 are constructed by a structure as represented in FIG. 8. In this apparatus of FIG. 8, the signal to be supplied to the first-stage reception mixer 15 is directly supplied to the quadrature modulator 31 so as to directly modulate the signal to be transmitted. In this case, it is assumed that the frequency of the signal outputted from the oscillator 41 is selected to be from 864 MHz to 889 MHZ in the reception (RX) time slot, whereas it is selected to be from 890 MHz to 915 MHz in the transmission (TX) time slot. In this case, since the intermediate frequency signal of 116 MHz is no longer required in the signal transmission system, the oscillator 43 is not required. As a result, the entire arrangement of this apparatus could be made simpler.
However, since the oscillating frequency of the signal derived from the oscillator 41 is identical to the frequency of the transmission signal outputted from the quadrature modulator 31, the oscillating operation of the oscillator 41 is swung by the carrier of this transmission signal, so that this oscillating operation would become unstable.
To avoid this difficulty, it would also be conceivable to arrange this apparatus in a manner shown in FIG. 9. In this alternative structure of FIG. 9, the circuit arrangement of the transmission system is constructed similar to that of FIG. 5, whereas the circuit arrangement of the reception system is constructed different from that of FIG. 5. That is, the intermediate frequency signal having the frequency of 71 MHz produced from the first-stage reception mixer 15 is not converted into the second intermediate frequency signal having the frequency of 6 MHz, but is directly supplied into the quadrature demodulator 21. As a consequence, the frequency of the signal supplied from the oscillator 44 into the quadrature demodulator 21 is also selected to be 7 MHz. After an I signal component and a Q signal component, which are outputted from the quadrature demodulator 21 are modulated by sigma/delta modulators 61a and 61b, respectively, only signal components having predetermined frequency bands are separated by digital filters 62a and 62b, which will then be supplied to the baseband process unit 2.
With such a circuit arrangement, since the second intermediate frequency signal having the frequency of 6 MHz in the reception system is no longer required, the oscillator 44 is not necessary.
However, three oscillators are still required in this example.